Scavenging and active predation in generalist predators: A mesocosm study employing DNA-based gut content analysis

Scavenging presumably is common among invertebrate generalist predators, however, only few studies have specifically dealt with the consumption of dead prey. Here consumption of dead and living prey within a guild of generalist predators was investigated using mesocosm experiments and DNA-based gut content analysis. A community of predators including carabids, staphylinids and spiders was released into 0.2 m² mesocosms, planted with wheat and infested with grain aphids Sitobion avenae. At predator release, freshly killed bird cherry-oat aphids, Rhopalosiphum padi, were provided as carrion prey at the soil surface. Two days after predator release, predators were collected and their gut content screened for DNA of both aphid species using multiplex PCR. Comparing detection frequencies of DNA from living and dead prey, we found that all predators frequently consumed dead aphids, except for the lycosid spider Trochosa ruricola which exclusively fed on S. avenae. In contrast, in the tetragnathid spider Pachygnatha degeeri, supposed to feed mainly on living prey, detection rates of R. padi DNA were high. Our findings support previous assumptions that many generalist predators are facultative scavengers. This needs to be considered for the interpretation of field-derived data using approaches of gut content analyses as well as when assessing predators’ pest control potential.     

Impact of ecologically different earthworm species on soil water characteristics

A laboratory experiment was performed to assess the impact of ecologically different earthworm species on soil water characteristics, such as soil tension, water content, and water infiltration rate. Three earthworm species (Lumbricus rubellus, Aporrectodea caliginosa, Lumbricus terrestris) were exposed in soil columns (diameter 30 cm, height 50 cm) for 100 days with a total fresh earthworm biomass of 22.7 ± 0.4 g per column, each in duplicate. Each column was equipped with tensiometers at 10 and 40 cm and FD-probes at 10 cm depth, to continuously measure the temporal development of soil tension and soil moisture. Additionally, 30 g of sieved and rewetted horse manure was placed on the soil surface as a food source. Precipitation events (10 mm) were simulated at day 28 and day 64. At the end of the experiment the water infiltration rate and the runoff at 55 cm depth were determined.The results showed considerable evidence, that ecologically different earthworms modify soil water characteristics in different ways. The anecic L. terrestris and the endogeic A. caliginosa showed the tendency to enhance the drying of the topsoil and subsoil. Their intensive and deep burrowing activity might enhance the exchange of water vapor due to a better aeration in soil. In contrast, the epigeic L. rubellus tended to enhance the storage of soil moisture in the topsoil, which might be linked to lower rates of litter loss from soil surface and thus a thicker litter layer remaining. A. caliginosa led to considerable higher water infiltration rates and faster water discharges in the subsoil, relative to the other species, probably due to a high soil dwelling activity.     

Deciduous woodland exposed to elevated atmospheric CO2 has species-specific impacts on anecic earthworms

Elevated atmospheric CO₂ induced reductions in litter quality can adversely affect earthworms. However, this understanding is based on laboratory rather than field research and relates to single earthworm and tree species. Here earthworm populations were investigated under Alnus glutinosa, Betula pendula, and Fagus sylvatica in a Free Air Carbon dioxide Enrichment field experiment. Litters from this experiment were also fed to Lumbricus terrestris L. at two rates with live weight change and cast properties assessed. Elevated CO₂ (580 ppmv) reduced litter N (−12%) with a corresponding increase in C:N ratio, especially for A. glutinosa. In the field, elevated CO₂ caused a shift in overall population composition, mainly characterised by reduced anecic biomass (–25%); endogeic and epigeic species were less affected. CO₂ effects on total biomass were most pronounced for A. glutinosa (e.g. field total biomass −47% vs. −11% overall). Growth of L. terrestris was lower when fed elevated CO₂ litter (−18%), although increased inputs of A. glutinosa litter mitigated this effect. In mesocosms, fresh cast respiration was lower (−14%) for elevated CO₂ litter, an effect more pronounced for A. glutinosa (−24%). When normalised for C content, elevated CO₂ effects on cast respiration were again negative and most marked for A. glutinosa litter. Litter N concentration, and possibly ease of litter mineralisation were factors affecting litter resource quality Litter N and P concentrations varied with A. glutinosa > B. pendula > F. sylvatica; F. sylvatica had the highest cellulose content. Field earthworm biomass was higher under A. glutinosa compared with B. pendula and F. sylvatica (+17 and +70%, respectively); live weight increased with A. glutinosa litter in the feeding trial almost three times more than for B. pendula, whereas it decreased for F. sylvatica. Cast respiration was highest for A. glutinosa, intermediate for B. pendula (ca. −36%) and lowest for F. sylvatica (ca. −78%). Earthworm responses to elevated CO₂ were complex, being characteristic of individual tree and earthworm species; responses were more adverse for trees with higher quality litter and for anecic earthworms.     

Deep-burrowing earthworm additions changed the distribution of soil organic carbon in a chisel-tilled soil

We investigated the influence of earthworms on the three-dimensional distribution of soil organic carbon (SOC) in a chisel-tilled soil. By burrowing, foraging, and casting at the surface and throughout the soil, anecic earthworms such as Lumbricus terrestris L. may play a major role in regulating the spatial distribution of organic matter resources both at the surface and within the soil. In the fall of 1994, we manipulated ambient earthworm communities, which were without deep burrowing species, by adding 100 earthworm individuals m⁻² in spring and fall for 3 years. Overall, the biomass of L. terrestris was increased with earthworm additions and total earthworm biomass declined compared with ambient control treatments. To investigate the spatial variability in soil organic carbon due to this shift in earthworm community structure, we sampled soil on a 28×24 cm grid from the surface to 40 cm in four layers, 10 cm deep. Samples were analyzed for total carbon. We found that additions of anecic earthworms significantly increased average soil organic carbon content from 16.1 to 17.9 g C kg⁻¹ for the 0–10 cm soil, and from 12.4 to 14.7 g kg⁻¹ at 10–20-cm depth, and also changed the spatial distribution of soil organic carbon from uniform to patchy, compared with the ambient treatment.     

Allyl isothiocyanate: an alternative chemical expellant for sampling earthworms

Allyl isothiocyanate (AITC), a natural breakdown product of glucosinolates in many Cruciferae and a component imparting the sharp taste to mustard, was tested for its effectiveness as a chemical expellant for sampling earthworms. Testing was performed in an arable field with earthworm populations dominated by Lumbricus terrestris Linnaeus, 1758 and Aporrectodea tuberculata (Eisen, 1874). The optimal concentration was found by comparing concentrations ranging from 5 to 250 mg l⁻¹ in water. Total biomass and numbers of earthworms collected increased hyperbolically with increasing AITC concentration, with the highest biomass and numbers collected using 100 mg l⁻¹ AITC. Biomass and numbers of earthworms collected using 250 mg l⁻¹ AITC, but not 150 or 200 mg l⁻¹ AITC, were significantly less than with 100 mg l⁻¹ AITC. Less earthworm biomass was collected by hand sorting than with chemical expulsion using 100 mg l⁻¹ AITC, but the number of earthworms collected by the two methods were not different. A comparison of hand sorting and 100 mg l⁻¹ AITC expulsion using analysis of similarities (ANOSIM) showed that the two methods produced samples differing in the distribution of both numbers and biomass of species-by-size classes. Hand sorting collected more of the smallest size class of L. terrestris and the largest size classes of A. tuberculata than AITC expulsion, whereas AITC expulsion recovered more of the largest size classes of L. terrestris than hand sorting. When 100 mg l⁻¹ AITC expulsion was compared with chemical expulsion using 200 mg l⁻¹ formalin, no differences were found in the total number, total biomass or in the species-by-size class distribution of the earthworms collected. This suggests that the AITC method may be a favorable alternative to formalin expulsion for sampling earthworms. Further studies under other environmental conditions and with other species of earthworms are warranted to establish its general utility.     

Miscanthus sinensis and wild flowers as food resources of Lumbricus terrestris L.

Senescent leaves of Miscanthus sinensis contained 36% soluble polysaccharides, 26% cellulose and had a C/N ratio of 45. In 11 wild flower species contents of soluble polysaccharides (21–30%), cellulose (3–16%) and C/N ratio (13–31) were lower. Decomposing leaves of M. sinensis lost weight at a rate of 0.002 day⁻¹, increased the C/N ratio from 45 to about 100, the bacterial biomass from 0.4 to 1 μg C mg⁻¹ dry weight, and decreased the tensile strength from 35 to 10 N. The withdrawal rate of Lumbricus terrestris with senescent leaves of M. sinensis was 30 mg g⁻¹ week⁻¹; the feeding rate was lower. With most senescent wild flowers withdrawal and feeding rates were higher. During decomposition of M. sinensis withdrawal rates increased to about 90, and feeding rates to about 30 mg g⁻¹ week⁻¹. The rates were not related to soluble polysaccharides, cellulose, acid-insoluble residue, C/N ratio and the presence of trichomes on the leaves. The abundance of L. terrestris decreased in a meadow turned into a field of M. sinensis from 55 to 26 earthworms m⁻² and increased in a rotational maize field turned into wild flower strips from 28 to 46 earthworms m⁻². The species richness of earthworms decreased with M. sinensis from 7.2 to 4.7 and increased with wild flowers from 4.7 to 6.7 species per sampling unit.     

Assessing exotic plant species invasions and associated soil characteristics: A case study in eastern Rocky Mountain National Park,Colorado, USA,using the pixel nested plot design

Rocky Mountain National Park (RMNP), Colorado, USA, contains a diversity of plant species. However, many exotic plant species have become established, potentially impacting the structure and function of native plant communities. Our goal was to quantify patterns of exotic plant species in relation to native plant species, soil characteristics, and other abiotic factors that may indicate or predict their establishment and success. Our research approach for field data collection was based on a field plot design called the pixel nested plot. The pixel nested plot provides a link to multi-phase and multi-scale spatial modeling-mapping techniques that can be used to estimate total species richness and patterns of plant diversity at finer landscape scales. Within the eastern region of RMNP, in an area of approximately 35,000 ha, we established a total of 60 pixel nested plots in 9 vegetation types. We used canonical correspondence analysis (CCA) and multiple linear regressions to quantify relationships between soil characteristics and native and exotic plant species richness and cover. We also used linear correlation, spatial autocorrelation and cross correlation statistics to test for the spatial patterns of variables of interest. CCA showed that exotic species were significantly (P < 0.05) associated with photosynthetically active radiation (r = 0.55), soil nitrogen (r = 0.58) and bare ground (r = −0.66). Pearson's correlation statistic showed significant linear relationships between exotic species, organic carbon, soil nitrogen, and bare ground. While spatial autocorrelations indicated that our 60 pixel nested plots were spatially independent, the cross correlation statistics indicated that exotic plant species were spatially associated with bare ground, in general, exotic plant species were most abundant in areas of high native species richness. This indicates that resource managers should focus on the protection of relatively rare native rich sites with little canopy cover, and fertile soils. Using the pixel nested plot approach for data collection can facilitate the ecological monitoring of these vulnerable areas at the landscape scale in a time- and cost-effective manner.     

Lumbricus terrestris L. does not impact on the remediation efficiency of compost and biochar amendments

The aim of this study was to test the impact of compost and biochar, with or without earthworms, on the mobility and availability of metals, and on the growth of grass to re-vegetate contaminated soil from the Parys Mountain mining site, Anglesey. We also determined if the addition of earthworms compromises remediation efforts.In a laboratory experiment, contaminated soil (1343 mg Cu kg^?1, 2511 mg Pb kg^?1 and 262 mg Zn kg^?1) was remediated with compost and/or biochar. After 77 days Lumbricus terrestris L. earthworms were added to the treatment remediated with both compost and biochar, and left for 28 days. L. terrestris was not able to survive in the biochar, compost or unamended treatments. A germination and growth bioassay, using Agrostis capillaris (Common Bent) was then run on all treatments for 28 days.The combination of biochar and compost decreased water soluble Cu (from 5.6 to 0.2 mg kg^?1), Pb (from 0.17 to less than 0.007 mg kg^?1) and Zn (from 3.3 to 0.05 mg kg^?1) in the contaminated soil and increased the pH from 2.7 to 6.6. The addition of L. terrestris to this treatment had no effect on the concentration of the water soluble metals in the remediated soil.The compost was the only treatment that resulted in germination and growth of A. capillaris suitable for re-vegetation purposes. However, the combination of compost and biochar (with or without L. terrestris) produced the lowest concentrations of Cu (8 mg kg^?1) and Zn (36 mg kg^?1) in the aboveground biomass, lower than the compost treatment (15 mg Cu kg^?1 and 126 mg Zn kg^?1).The addition of biochar and compost both separately and as co-amendments was effective in reducing the mobility and availability of metals. The addition of L. terrestris did not re-mobilise previously sequestered metals.     

Effects of land use changes on winter-active Collembola in Sanjiang Plain of China

Sanjiang Plain is the largest concentrated area of freshwater wetlands in China, however nearly 80% of these freshwater wetlands were drained or reclaimed in the past 50 years. It is important to know whether wetlands reclamation would affect soil invertebrates, especially the winter-active invertebrates. During November 2011 to April 2012, we used pitfall traps and in-field direct observation methods to study the activity of collembolans in wetland, and a reclaimed forest plantation and soybean field. In total, 3465 collembolans were captured and identified to 8 species from 6 families. Desoria sp. 1, Desoria sp. 2 and Desoria sp. 3 were the three dominant species. Collembolan abundance and assemblages were significantly affected by the land use changes. The results showed that (1) Collembola captured by pitfall traps showed a highest abundance in wetland, with ∼50% decreased abundance in soybean field, and ∼75% decreased in the forest plantation. (2) Collembola activity changed during the winter season, their activity significantly increased from early winter to late winter; with a peak in March in all three land use types. (3) Collembola assemblages were affected by land use changes with a significant decrease of frequency of Desoria sp. 3 and a significant increase of frequency of Desoria sp. 2 in soybean field and forest plantation. (4) Collembolan densities on the snow surface usually peaked between 1400 h and 1500 h, and decreased quickly when the temperature dropped below freezing. Mean abundance reached 119 individuals m⁻² on the snow surface in wetland, 152 individuals m⁻² in soybean field, and 64 individuals m⁻² in forest plantation. All collembolans moved up and down through the snow profile depending on temperature, no collembolans were found on the snow surface in the evening. Our study indicated that the reclamation of wetland resulted in a significant decrease of abundance and a different assemblage of winter active Collembola in Northeast China, but land use changes did not change their pattern of activity: Collembolans were rarely active in early and middle winter and mostly active in late winter. Their daily densities on the snow surface fluctuated according to both air temperatures and land use types.     

Remote sensing of crop residue cover and soil tillage intensity

Management of plant litter or crop residues in agricultural fields is an important consideration for reducing soil erosion and increasing soil organic C. Current methods of quantifying crop residue cover are inadequate for characterizing the spatial variability of residue cover within fields or across large regions. Our objectives were to evaluate several spectral indices for measuring crop residue cover using satellite multispectral and hyperspectral data and to categorize soil tillage intensity in agricultural fields. Landsat Thematic Mapper (TM) and EO-1 Hyperion imaging spectrometer data were acquired over agricultural fields in central Iowa in May and June 2004. Crop residue cover was measured in corn (Zea mays L.) and soybean (Glycine max Merr.) fields using line-point transects. Spectral residue indices using Landsat TM bands were weakly related to crop residue cover. With the Hyperion data, crop residue cover was linearly related to the cellulose absorption index (CAI), which measures the relative intensity of cellulose and lignin absorption features near 2100 nm. Coefficients of determination (r²) for crop residue cover as a function of CAI were 0.85 for the May and 0.77 for the June Hyperion data. Three tillage intensity classes, corresponding to intensive (<15% residue cover), reduced (15–30% cover) and conservation (>30% cover) tillage, were correctly identified in 66–68% of fields. Classification accuracy increased to 80–82% for two classes, corresponding to conventional (intensive + reduced) and conservation tillage. By combining information on previous season's (2003) crop classification with crop residue cover after planting in 2004, an inventory of soil tillage intensity by previous crop type was generated for the whole Hyperion scene. Regional surveys of soil management practices that affect soil conservation and soil C dynamics are possible using advanced multispectral or hyperspectral imaging systems.     

Temporal variations of soil arthropods community structure in El Pedregal de San Ángel Ecological Reserve,Mexico City,Mexico

Community structure of soil arthropods associated to Pittocaulon (Senecio) praecox was investigated in two sites with different vegetal cover in El Pedregal de San Ángel Ecological Reserve, South of Mexico City during thirteen-month period. The relationship of composition and abundance with soil variables was studied. A total of 101 483 specimens belonging to 28 taxa and 191 morphospecies were collected in both sites (Open site: Abundance: 60,369, 27 taxa and 181 morphospecies; Closed site: 41 114, 26 taxa and 179 morphospecies), the value of Shannon's diversity index was marginally higher in Closed site (H′ = 3.67) than in Open site (H′ = 3.59), according to a modified t-test for comparing indexes (t₉₁₄₁₈ = 9.6946; p < 0.001). Most abundant groups in both sites were Cryptostigmata and Prostigmata, with variations during the study period. The highest amount of specimens was recovered during the rainy season (May–October). The most abundant guilds recorded in both sites were predators, fungivores, detritivores and herbivores. The composition and trophic guilds of edaphic arthropods in the sites were affected by the microhabitat characteristics due to soil characteristics. According to a two-way ANOVA test, a significant effect due to site (F_1,286 = 17.85, p < 0.001) and month (F_12,286 = 22.68, p < 0.001) on the density of edaphic arthropods was recorded, but no significant differences in abundance were found when the interaction between both variables (F_12,286 = 0.77, p > 0.05) was considered. The RDA shows that the soil variables explain 23.5% of the variation observed in the distribution of species, and that variation is related mainly to the presence of K, P, Na and Mg, and groups of predator arthropods are more related to the aforementioned variables, while detritivorous groups are more related to MO, N contents, relative humidity and pH. The microenvironmental characteristics recorded in both sites can explain differences in composition and abundance of soil arthropods.     

Leaf litter is the main driver for changes in bacterial community structures in the rhizosphere of ash and beech

The rhizosphere and the surrounding soil harbor an enormous microbial diversity and a specific community structure, generated by the interaction between plant roots and soil bacteria. The aim of this study was to address the influences of tree species, tree species diversity and leaf litter on soil bacterial diversity and community composition. Therefore, mesocosm experiments using beech, ash, lime, maple and hornbeam were established in 2006, and sampled in October 2008 and June 2009. Mesocosms were planted with one, three or five different tree species and treated with or without litter overlay.Cluster analysis of DGGE-derived patterns revealed a clustering of 2008 sampled litter treatments in two separated clusters. The corresponding treatments sampled in 2009 showed separation in one cluster. PCA analysis based on the relative abundance of active proteobacterial classes and other phyla in beech and ash single-tree species mesocosm indicated an effect of sampling time and leaf litter on active bacterial community composition. The abundance of next-generation sequencing-derived sequences assigned to the Betaproteobacteria was higher in the litter treatments, indicating a higher activity, under these conditions. The Deltaproteobacteria, Nitrospira and Gemmatimonadetes showed an opposite trend and were more active in the mesocosms without litter. The abundance of alphaproteobacterial sequences was higher in mesocosms sampled in 2009 (P = 0.014), whereas the Acidobacteria were more active in 2008 (P = 0.014). At the family level, we found significant differences of the litter vs. non-litter treated group. Additionally, an impact of beech and ash as tree species on soil bacterial diversity was confirmed by the Shannon and Simpson indices. Our results suggest that leaf litter decomposition in pH-stable soils affect the soil bacterial composition, while tree species influence the soil bacterial diversity.     

Field margins and management affect settlement and spread of an introduced dew-worm (Lumbricus terrestris L.) population

To study the feasibility of earthworm introduction for increasing the macroporosity and permeability of arable heavy clay, deep-burrowing earthworms (Lumbricus terrestris L.) were inoculated into a tile drained experimental field in Jokioinen, S-W Finland in autumn 1996. Inoculation with the Earthworm Inoculation Unit technique was at the up-slope end of the field, in the field margins under permanent grass, and inside the four 0.46 ha plots of the field. The experiment was monitored on three occasions. In 1998 the L. terrestris population had persisted in low numbers only in field and plot margins. By 2003, when the field had been under set-aside grass for three years, density had grown in the margins and L. terrestris were also found inside the field at a very low density. The third monitoring was in autumn 2009, after a further four years as set-aside and a subsequent division of the field into no-till and ploughing management, and looked at the effects of management (margins, no-till, ploughing), distance from the inoculation and sub-drainage on L. terrestris abundance. The abundance displayed a clear gradient over the field, declining from 14 ind. and 18 g m^?2 at 5–9 m from inoculation, to 1 ind. and 2 g m^?2 at 56–60 m distance. Margins had the highest abundances (16 ind. and 32 g m^?2), followed by no-till (4 ind. and 4 g m^?2) and ploughing (1 ind. and 1 g m^?2). Abundances were significantly higher above the tiles than between them (P < 0.05). The results demonstrate the importance of no-till and sub-drain line habitats as settlement supports for the inoculated population. Field margins proved to be decisive for inoculation success, by providing bridgeheads for population establishment and later by acting as source areas for the colonisation of the field. This finding highlights the general importance of field margins in the dispersal ecology of earthworms in arable landscapes.     

Spatial dissociation between two endogeic earthworms in the Colombian “Llanos”

Although there has been a growing interest in the study of soil fauna spatial distribution during the past decade, the identification of the environmental driving factors behind the population patterning are difficult to highlight. Soil physico-chemical heterogeneity is partly responsible for structuring the population. However, the available statistical analyses show that the proportion of the population spatial variance that can be ascribed to soil habitat variability is modest. We studied the spatial distribution of two medium-sized endogeic earthworm species (Andiodrilus sp. and Glossodrilus sp.) and the spatial segregation between them. The survey was undertaken in a native savanna and a grass-legume pasture in the Colombian “Llanos”. The presence of spatial dependence in the data (i.e. earthworm counts) was tested using two different approaches: the Spatial Analysis using Distance IndicEs (SADIE) analyses and cross-coregionalization. The SADIE index allowed for testing the spatial association or dissociation between earthworm counts. The spatial organization of both species was well structured in the natural savanna while they were randomly distributed in the pasture in almost all sampling dates. When the spatial distribution was different from randomness it was always aggregated irrespective of the land-use system. There was no absolute stable spatial pattern in the natural savanna although a general pattern seemed to emerge. On the contrary, no pattern was observed in the pasture. Both species displayed opposite spatial distributions (P < 0.05) that were of different intensity depending on the sampling date. The presence of opposite patches and gaps suggests the presence of a competitive exclusion phenomenon (at least spatial) that deserves further investigations.     

Role of soil water content in the carbon and nitrogen dynamics of Lumbricus terrestris L. burrow soil

We evaluated the role of soil water content in controlling C and N dynamics within the drilosphere created by the anecic earthworm Lumbricus terrestris (L.). Mesocosms (volume = 3.1 l) were each amended with corn litter and three earthworms. Control treatments received no earthworms and no other earthworm species were present in the soil. WET and DRY treatments received a total of 9.25 cm and 3.25 cm of water, respectively. Water was added on weeks 1, 3, 7, and 10 at a rate of 2.0 cm per mesocosm for WET treatments and 0.5 cm per mesocosm for DRY treatments. Mesocosms were sampled destructively after incubation at 18–20 °C for 0, 3, 7, and 13 weeks. The water content of WET burrow soil ranged from 0.12 g g⁻¹ to 0.18 g g⁻¹ and was significantly higher than in the DRY treatment throughout the incubation period. The live weight of earthworms was significantly higher in the WET treatment only on week 13, whereas litter consumption was significantly lower in the DRY treatment for week 13. Carbon mineralization, measured as CO₂ evolved after a 24-h incubation, was consistently higher in WET than in DRY burrow soil. Effects of differences in soil water content were also apparent for biomass C and metabolic quotient. Soil water content did no affect the total C concentration of burrow soil. DRY burrow soil had consistently lower levels of nitrate than WET soil throughout the experiment. Lower levels of ammonium and inorganic N were observed for WET burrow soil on weeks 3 and 7. Water content did not have a significant effect on burrow soil total N. We concluded that the water content of the drilosphere affects both C and N dynamics and can affect the speciation of inorganic N; yet, the effects of soil water content do not appear to result from differences in the feeding activities of anecic earthworms.     

Mechanisms of short-term soil carbon storage in experimental grasslands

We investigated the fate of root and litter derived carbon in soil organic matter and dissolved organic matter in soil profiles, in order to explain mechanisms of short-term soil carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment between 2002 and 2004. In addition to the main experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with ¹³C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g C m^?2 and 263 g C m^?2 to 20 cm depth, while 71 g C m^?2 and 393 g C m^?2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 115 g C m^?2 and 156 g C m^?2 of soil organic carbon were derived from C4 plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of 97 g C m^?2 that was newly stored in the same soil depth, whereas with double litter this clearly exceeded the stored amount of 75 g C m^?2. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution. However, the total flow of dissolved organic carbon was not sufficient to explain the observed carbon storage in deeper soil layers, and the existence of additional carbon uptake mechanisms is discussed.     

Vertical distribution of microbial communities under the canopy of two legume bushes in the Tehuacán Desert,Mexico

Prosopis laevigata and Parkinsonia praecox are the most abundant perennial shrubs in the Tehuacán Desert, forming ’islands of fertility’ that dominate the alluvial terraces. Both species exhibit very similar phenology, with the timing of litter foliage being the only difference between them. P. praecox litter occurs shortly after the rains, while P. laevigata maintains its leaves until the next wet season. As degradable organic matter (OM) is one of the leading factors determining soil biota composition and activity, because of the OM provided by littering, we expected that the vertical distribution of the microbial community in the vicinity of the root zone of P. praecox would be higher in comparison to P. laevigata. One soil sampling was performed; during the rainy season in August, soil samples were collected from a 0–50-cm depth at 10-cm intervals, in the vicinity of the root canopy of four individual plants of each species and the interspaces between them. Soil moisture, organic matter, and counts of bacteria and fungi under shrubs were found to decrease from the upper to deeper layers. Respiratory activity was higher in the deeper layers (p < 0.01) in all three sampling sites. Total bacterial, fungal, and heterotrophic diazotrophs were found to be significantly (p < 0.001) more numerous under shrubs than in the interspace soil. No nitrogen-fixing bacteria were isolated from interplant soils in comparison to the soil samples collected beneath the shrubs. Heterotrophic diazotrophs significantly (p < 0.01) reduced more acetylene under P. praecox (29.0 nmol/g soil) than under P. laevigata (20.1 nmol/g soil). Although the microbial numbers were unaffected by differences in plant phenology, greater nitrogenase activity under P. praecox may influence nitrogen distribution in this arid environment. Due to the fact that only one sampling was undertaken, this study elucidates the differences in the microbial community between the two shrubs, but the dynamics in the above community could not be shown.     

Arable weeds,cover crops,and tillage drive soil microbial community composition in organic cropping systems

Cover crops have traditionally been used to reduce soil erosion and build soil quality, but more recently cover crops are being used as an effective tool in organic weed management. Many studies have demonstrated microbial community response to individual cover crop species, but the effects of mixed species cover crop communities have received less attention. Moreover, the relationship between arable weeds and soil microbial communities is not well understood. The objective of this study was to determine the relative influence of cover crop diversity, early-season weed communities, and tillage on soil microbial community structure in an organic cropping system through the extraction of fatty acid methyl esters (FAMEs). A field experiment was conducted between 2009 and 2011 near Mead, NE where spring-sown mixtures of zero (control), two, and eight cover crop species were included in a sunflower–soybean–corn crop rotation. A mixture of four weed species was planted in all experimental units (excluding the no-cover control), and also included as an individual treatment. Cover crops and weeds were planted in late-March, then terminated in late-May using a field disk or sweep plow undercutter, and main crops were planted within one week of termination. Three (2009) or four (2010–11) soil cores were taken to a depth of 20 cm in all experimental units at 45, 32, and 25 days following cover crop termination in 2009, 2010, and 2011, respectively. Total FAMEs pooled across 2009 and 2010 were greatest in the two species mixture–undercutter treatment combination (140.8 ± 3.9 nmol g⁻¹) followed by the eight species mixture–undercutter treatment combination (132.4 ± 3.9 nmol g⁻¹). Abundance of five (2009 and 2010) and seventeen (2011) FAME biomarkers was reduced in the weedy treatment relative to both cover-cropped treatments and the no-cover control. In 2009 and 2010, termination with the undercutter reduced abundance of most actinomycete biomarkers while termination with the field disk reduced abundance of C18:1(cis11) and iC16:0. Canonical discriminant analysis of the microbial community successfully segregated most cover crop mixture by termination method treatment combinations in 2009 and 2010. Microbial communities were most strongly influenced by the presence and type of early-spring plant communities, as weeds exerted a strong negative influence on abundance of many key microbial biomarkers, including the AMF markers C16:1(cis11) and C18:1(cis11). Weeds may alter soil microbial community structure as a means of increasing competitive success in arable soils, but this relationship requires further investigation.     

Darwin's earthworms revisited

Down House was Charles Darwin's home from 1842 until his death in 1882 and where he wrote “The Formation of Vegetable Mould through the Action of Worms”. The work described here is based upon passages from this book and from further observations on earthworms in this area. General observations were made in addition to systematic sampling in areas selected either from signs of earthworm activity, habitat type or in direct relation to Darwin's documented work.Greatest species richness (n = 9) was found in Middle Field. Greatest earthworm density was present in Darwin's Kitchen Garden (715 m⁻²) with the largest associated biomass (261 g m⁻²). Aporrectodea longa was the most abundant species. Lumbricus terrestris, described by Darwin in terms of its behaviour, but not directly named, was located in relatively low numbers, but its diagnostic middens and associated burrows were easily detected. Earthworms associated with Darwin's cinder and chalk application experiments were also examined. In total, 19 of Britain's 28 earthworm species were located within the nominated World Heritage Site.     

Spatial distribution of rock fragments on steep hillslopes in karst region of northwest Guangxi,China

Rock fragments (> 5 mm in diameter) at the soil surface and within the topsoil have a large effect on the intensity of various hydrologic and geomorphic processes. However, little information is available on the spatial distribution of rock fragments in subtropical regions. The objective of this paper was to investigate the relationship between the spatial distribution of rock fragments and landforms on two different steep karst hillslopes in northwest Guangxi, southwest China. On the first hillslope (a disintegrated landslide failure) with the presence of several large rock outcrops (> 2 m in height), the spatial distribution of rock fragment cover had no obvious relationship with topographic position except that the mean cover percentage of small rock fragments (5–20 mm) decreased from bottom to top. On the second hillslope (an avalanche slope) without the presence of large rock outcrops, the mean total rock fragment cover (5–600 mm) increased from bottom (5%) to top (21%) with decreasing variability and rock fragments with various sizes (5–20, 20–75, and 75–250 mm) showed a similar increasing trend. The mean total rock fragment cover increased linearly with slope gradient on the second hillslope and tended to increase and then decrease with gradient but their relationship was not obvious on the first hillslope. This indicated that the spatial distribution of surface rock fragment cover had a close relationship with the presence of large rock outcrops and slope gradient. However, the median diameter (D₅₀) of the surface rock fragments had an increase–decrease trend with slope gradient but there was no obvious relationship on both hillslopes with low overland flow. Therefore, the dominant factor for the spatial distribution of rock fragment cover and size at the soil surface might not be soil erosion by water, but slope gradient, vegetation and geomorphologic condition of the slope. The mean total volumetric rock fragment content (5–250 mm) within the topsoil (10–20 cm thick) increased linearly from bottom (16%) to top (39%) with slope gradient on the first hillslope, and had a logarithmic increase from bottom (10%) to top (27%) with gradient on the second hillslope. This suggested that rock fragment content within the topsoil was mostly controlled by slope gradient and topographic positions and had not a close relationship with the presence of large rock outcrops.